1. Field of the Invention
This invention relates to moisture responsive sensors, and more particularly to the use in such sensors of an organic polymer film with a low hydrophylic characteristic as a moisture-sensitive element.
2. Description of the Prior Art
Prior art organic polymer moisture-sensitive elements use an organic polymer such as celluloseacetatebutyrate, cellulloseacetatepropyonate, polyimide, or polyimideamide as a moisture-sensitive materials and utilizes an electrical capacitance change in a moisture-sensitive film formed of such moisturesensitive material to detect humidity, as disclosed in Published Japanese Patent Application No. 62-88951.
Such moisture-sensitive elements described above are highly hydrophilic and accordingly a large amount of water is absorbed by such elements. The absorbed water is strongly bonded to the polymer and remains in the element. For this reason, if the element is used at a high temperature and high relative humidity atmosphere, e.g. at 40.degree. C. and 90% for a long time, the output value thereof drifts or long-term stability is damaged.
Also, in the highly hydrophilic prior art organic polymer elements a moisture-sensitivity characteristic difference (hysteresis) between moisture absorption and desorption processes is decreased at a low-temperature side and increased at a high-temperature side, thereby causing delay in sensor response.
Further, if such a prior art element is used in a low humidity atmosphere for a long time the hysteresis will become larger. Even in a favorable atmosphere, a continuous use of the element for a long time will cause its capacitance ratio to be change, thus resulting in deteriorating of long-term stability of the element.
Furthermore, moisture condensation or water dipping will also cause the output of the element to drift.
For example, celluloseacetatebutyrate (hereinafter simply referred to as "CAB") having 17% of butyryl base typically used as moisture-sensitive material in the above-mentioned type of conventional moisture-sensitive element has a water sorption amount of approximately 70-90 mg/g in an equilibrated condition at a temperature of 30.degree. C. and a relative humidity of 90% RH.
Such moisture-sensitive element having a moisture-sensitive film made of CAB presents 2-4% RH of hysteresis (difference in moisture sensitivity characteristic between moisture absorption and desorption processes) at about 25.degree. C. and at measuring intervals of 5 minutes. Therefore, it is not possible to reduce the hysteresis to less than 1% RH, as considered ideal. Also, the hysteresis is temperature dependent and increased by 2-3% RH at about 40.degree. C. compared with the value derived at 25.degree. C. if other conditions are the same.
Further, if the above-mentioned type of moisture-sensitive element is left in the same atmosphere for a long time, the effect of humidity to which the element has previously been exposed causes a change in the hysteresis and the moisture sensitivity characteristic. Specifically, if the element is left, e.g., at a relative humidity of 11% RH and a room temperature for a long time, the hysteresis is increased from an initial value of 2% RH to 4-6% RH. If the same element is left at a relative humidity of about 84% RH and a room temperature for a long time, the hysteresis is decreased from the initial value, i.e. 2% RH to about 1% RH. However, with respect to the moisture sensitivity characteristic, the output is increased by 5-7% RH in a relative humidity ranging from 10 to 90% RH, that is, a drift occurs in the output. Further, a ratio of an electrical capacitance value at a relative humidity of 90% RH to that at a relative humidity of 10% RH (C.sub.90 /C.sub.10) is fluctuated while the element is used for a long time.
Further, a high temperature and high humidity environment, moisture condensation, water dipping, a temperature and humidity cycle and so on also cause the moisture sensitivity characteristic of the element to drift largely in the positive direction.
These problems are caused by the fact that a water sorption amount (water adsorption ratio) of the moisture-sensitive material employed in the above-mentioned conventional moisture-sensitive element is too large to be an ideal capacitance type or impedance type moisture-sensitive element. Thus, interaction of water molecules absorbed in the moisture-sensitive material causes a change in response of the element and an equilibrated water sorption amount in a measured humidity atmosphere, thereby incurring a drift in the output of the element.